1. Field of the Invention
The invention relates generally to kick detection and monitoring in a borehole. More particularly, this invention relates to methods and systems for kick detection based on mud slowness.
2. Background Art
Well control is an important aspect of oil and gas exploration. When drilling a well in oil and gas exploration, various measures should be put in place to prevent injury to personnel and equipment associated with the drilling activities. For example, drilling fluids (mud) should have a proper weight to prevent the formation fluids from rushing into the wellbore. Even with all the precautions, penetration of unexpected high-pressure zones in the formation is unavoidable. When this occurs, high-pressure fluids typically flow into the wellbore, and the well is said to have taken a “kick.”
The pressure increase associated with a kick results from the influx of formation fluids (a liquid, a gas, or a combination thereof) from the higher-pressure formation into the wellbore. The high-pressure kick will propagate from the point of entry in the wellbore uphole (i.e., from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. These “blowouts” may result in catastrophic destruction of the drilling equipment and injury or death of personnel.
Kick is one of the biggest safety issues a driller faces. Every year, kick/blowout incidents cost the industry millions of dollars. While various measures (e.g., blowout preventers; BOP) are available that can reduce the risk of blowouts after a well takes a kick, it would be more effective if there were a way to predict or detect an impending kick and blowout. Early detection of an incipient kick can warn the driller and provide valuable time to plan a measure to kill the kick and to avoid a catastrophe. On the other hand, if a kick is detected late, it may be necessary to trip the drill string, to perform kick control measures such as circulating out the kick gas, or to activate the blowout preventers. All these measures are very costly. Even worse, if a kick is detected too late to have proper measures initiated, it could result in a blowout, leading to catastrophic damages to the equipment and/or personnel. Therefore, methods and systems that can monitor and detect a kick early are very valuable.
Most kick detection methods are based on monitoring the drilling fluid volume, flow rate, or pressure changes. For example, U.S. Pat. No. 6,371,204 issued to Singh et al. discloses a method for detecting a kick by carefully monitoring the fluid volumes that are being pumped into the drill string and being returned to the surface. By comparison or totaling of the calculated volume, it is possible to detect unwanted fluid flexes between the wellbore and an underground formation. The fluid volumes may be determined by monitoring the fluid levels, using a sonic or acoustic sensor that can provide a measure of the fluid interface.
Acoustic sensors may also be used in a different manner to detect a kick. For example, U.S. Pat. No. 4,273,212 issued to Dorr et al. discloses a method that uses acoustic sensors to monitor the distance from the rig to the bottom of the wellbore while drilling. An acoustic sensor at the surface transmits a signal into the mud column. The acoustic signal travels downhole and is reflected from the collar of the drilling assembly back to the rig. The distance between the rig and the bottom hole assembly (BHA) can be calculated and used to monitor the drilling process. If there is any influx of formation fluids (especially gases), the acoustic signals will be significantly altered. Thus, influx of formation fluids will produce aberrations in the signal travel times (hence, the calculated distances). Any such change may signal a kick.
U.S. Pat. No. 4,733,233 issued to Grosso et al. and U.S. Pat. No. 4,733,232 issued to Grosso disclose a method that uses a downhole acoustic source to produce acoustic signals and then uses two pressure transducers at the surface to sense annular acoustic waves in the returning mud flow and to sense annular acoustic waves in the drill string. A change in the amplitude of the annulus signal relative to the amplitude of the drill string signal is used to indicate the presence of a borehole fluid influx.
U.S. Pat. No. 5,154,078 issued to Codazzi discloses two methods for kick detection while drilling. The first method is based on the standing wave patterns generated by pressure oscillations of the drilling rig mud pumps. Such standing wave patterns form time sequences of maximum and minima, which may be monitored to derive the phase and period of the standing waves. A continuous increase in the phase difference between annulus and drill string standing waves may indicate a kick. The second method proposed in this patent uses acoustic signals from a downhole source near the bottom of the borehole. If there is a gas influx into the wellbore (annulus), the acoustic signals travel in the annulus at a different speed from that in the interior of the drill string. Thus, a difference in the arrival times exceeding a criterion may signify a kick.
Another approach is to monitor the pressure changes in the mud column. Most dangerous kicks result from gas influx. Once the gas flows into the wellbore, it will alter the hydraulic pressure of the mud column. Therefore, pressure sensors can provide a sensing means for monitoring the influx of gases. An example of this approach can be found in U.S. Pat. No. 6,176,323 issued to Weirich et al.
While various kick detection techniques exist, there is a continued need for improved methods and systems that can provide early kick warning to further improve the safety of subsurface exploration and production operations.